1. Field of the Invention
The present invention relates to a method of forming an oxide substance dielectrode thin film, and more particularly a method of manufacturing a ferroelectric capacitor using an oxide ferroelectric material.
2. Description of the Related Art
Material by using SrBi2Ta2O9, which is Bi based layer compound (hereinafter, a material in which composition is changed and a series of compound groups to which an addition substance is added or replaced are referred to as SBT) or Pb(Zr1−xTix)O3, which is titanic acid zirconate (hereinafter, a material in which composition of the compound is changed and a series of compound groups to which addition such as La or Ca is added are referred to as PZT) is currently put practical in use as a ferroelectric material used in a ferroelectric capacitor.
A memory using ferroelectric capacitors has an advantage in that an operation is available with 5V or below without an internal increase pressure circuit, which is essential for the above non volatile memory while the memory has the same function as a non volatile memory as represented in a flash memory or an EEPROM.
In particular, generally resistance of an electric field of a SBT lowers and the SBT is superior in a saturated characteristic for an electric filed of a residual polarization of the SBT compared with the resistance of the electric field and the saturated characteristic of a PZT as a ferroelectric material. Thereby, it is deemed that the SBT is suitable for a low voltage operation.
The residual polarization value of the PZT rapidly lowers with thinner film than film of 3000 Å. The PZT shows clearly film thickness independency as aforementioned. Therefore, the PZT has a drawback that practical in use of a device to operate with a voltage, which is below 5V is difficult. In contrast, since the film thickness dependency of the SBT lowers compared with the PZT, the SBT has an advantage that the SBT is easily applied to a device as purpose of low voltage operation.
In fact, when the film thickness is gradually thin as purpose of low voltage operation in the SBT, a big problem in which a resist pressure for an impressed voltage is rapidly decreased occurs. It is deemed that as this reason, a trough and peak on a surface of the SBT is greatly related as shown in FIG. 1.
FIG. 1 is a SEM photo of a ferroelectric capacitor in a conventional art, which sandwiches a SBT film by Pt electrodes. The trough and peaks on the surface reflects the trough and peak of the SBT film and an upper Pt electrode film goes into a space of the SBT film. Due to the troughs and peaks on the surface of the SBT, an extreme thin portion of a local area appears in the SBT film and an electric field is concentrated on that portion. As a result of this, breakdown voltage of the film is greatly decreased.
Roughness of the trough and peak on the surface of the SBT film is a common feature of a crystal structure of Bi based layer compound and roughness is ascribed to anisotropy of a crystal growing speed. In a conventional formation method of the SBT film, generally coking solution is made by a sol-gel method or by an organometallic decomposition method (MOD method), coking solution is coated on a substrate by a spin coat method, and an anneal step is perform at crystallization temperature. When a film with thickness around 150 nm is formed by this method, a difference between the trough and the peak of the film reaches 100 nm or more.
In general, when a ferroelectric film with desired thickness of 100 nm to 300 nm is formed by the spin coat method, a ferroelectric characteristic is improved and thereby any steps from a coating step to a crystallization heating step are repeated from twice to sixth times. When steps from the coating step to an organic solvent drying step, which have been reported are repeated, multiple spaces occur in the ferroelectric film at a time of a ferroelectric crystallization heating process and, therefore, there is a problem that a high quality film can not be formed. A metal organic substance can not be decomposed sufficiently in the heating process around 250° C. of which purpose is dry of organic solvent and in the film, the metal organic substance remains as it is. In that state, when the film is stacked and becomes thick, it is difficult to eliminate the metal organic substance from the organic film at a time of crystallization anneal after the film becomes thick and multiple spaces occur in the film from anisotropy of film contraction at a time of crystallization. As a result, leak resist pressure of the ferroelectric capacitor is reduced and a residual polarization value also lowers. Japanese Patent Application Laid-Open No. H8-340084 discloses that steps from a coating step to a drying step are repeated and therefore the aforementioned problem is apprehended. Similarly, Japanese Patent Application Laid-Open No. H9-69614 discloses that the multilayered film is formed in steps from the coating step to the drying step. Japanese Patent Application Laid-Open No. H9-153597 has a similar problem and proposes that a reduction pressure anneal method is used. However, cost for an apparatus is extremely expensive with respect to reducing a pressure of oxide with high temperature of 700° C. to 800° C. and, thereby the method is not suitable for mass manufacture.
There is a problem that only anneal step in a short time of a few minutes using a rapid heating method (RTA) can not obtain the ferroelectric characteristic sufficiently in a method to repeat a series of steps from the coating step to the heating step to decompose the organic substance or a series of steps from the coating step to the crystallization heating step (Japanese Patent Application Laid-Open No. H10-270646 discloses this method).
Though it is possible to suppress the growing of a grain by accelerating temperature increase rate using the RTA in some ranges and by dense crystallization nucleation of the ferroelectric film, there is a problem that a sufficient characteristic of the ferroelectric film can not be obtained when a heating time to crystallize the film is short. When the heating time in the RTA is simplify long to improve the film's characteristic, throughput becomes greatly worsen from a characteristic of an apparatus, a single wafer process and thereby, the method is not suitable for mass manufacture.